The present invention relates to a method of producing large sinter bodies of hard alloy (hard metal). More particularly, it relates to a method of producing hard alloy plungers and hard alloy matrixes which are used as pressing tools for the ultrahigh pressure synthesis of diamonds or cubic boron nitride as well as for the ultrahigh pressure sintering of polychrystalline diamond or polychrystalline cubic boron nitride.
Such hard alloy plungers and hard alloy matrixes are substantial components of the "belt" or "girdle" pressing apparatus. Other ultrahigh pressure pressing apparatuses are designed in accordance with the multiple plunger principle. They require no matrix. For example, the tetraeder pressing apparatus has four plungers and the cubic pressing apparatus has six plungers. In these pressing apparatuses the shape of the plunger is similar to the shape of the plungers in the above mentioned pressing apparatuses. Instead of truncated cones, they have a truncated triangular pyramid or a truncated square pyramid.
The production of hard alloy plungers and matrixes for smaller apparatuses which are sufficient for experimental synthesis does not pause significant problems in the prior art. For the industrial production of diamonds and cubic boron nitrides or for the above mentioned polychrystalline products, increasing greater pressing apparatuses are required with correspondingly greater plungers and matrixes.
They deal with plungers having a diameter of the pressing surface greater than 40 mm, a diameter of the basic surface greater than 80 mm, and a height greater than 60 mm. Correspondingly, the matrixes have the inner diameter greater than 40 mm, the outer diameter greater than 120 mm, and the height greater than 60 mm. Since the development of greater ultrahigh pressure pressing apparatuses has been maintained, an upper limit of the dimensions of such hard alloy plungers and matrixes at this time is not conceivable.
A typical example for large plunger and matrix are the plunger with a diameter of a pressing surface of 120 mm, the diameter of the basic surface 240 mm, and the height 180 mm. The corresponding matrixes have an inner diameter of approximately 120 mm, an outer diameter of approximately 360 mm, and a height of 120 mm. The greater the plunger and matrixes, the more problematic is the sintering process for their production.
Production of hard alloy parts formed by pressing and sintering of powder mixtures composed of hard carbide and binding metal. Plungers and matrixes for the ultrahigh pressure technique are preferably produced from mixtures of tungsten carbide and cobalt as binding metal.
The initial powders have fine grains, and by joint wet milling are further comminuted and thoroughly mixed. Subsequently, the powder mixture for the pressing process is prepared by granulation. For this purpose a granulating medium is utilized, which conventionally is composed of paraffin or wax and has a fraction of approximately 0.5 weight percent--2 weight percent. The pressing of larger "green products" (raw bodies) is performed preferably in a rubber form with a pressure of 100-200 MPa in cold isostatic pressing processes. By shaping for correcting the mass can be performed by material removal, during which the shrinking (linearly approximately 20%) produced during sintering is taken into consideration. In many cases however the shaping machining is performed after the presintering.
The presintering of the pressing parts is performed with increasing temperature. The end temperature amounts to approximately 970 K.-1,170 K. in a first part of the presintering, in the temperature region up to approximately 700 K the granulating binding medium is evaporated ("degrowth"). The gases (CO, CO.sub.2, H.sub.2 O, CH.sub.4) which escape in subsequent higher temperature region of the presintering, are produced under the alternating action of residual metal oxide with the carbon fraction and the oven atmosphere in the sinter body.
At the end of presintering, the sinter body has fine pores, and is significantly pure, or in other words it is released from steam or gas emitting residues.
Due to the presintering there is no high sintering shrinkage. The parts can be however well handled and mechanically worked to achieve the predetermined final dimensions in correspondence with the expected total shrinkage during subsequent sintering process per se.
The final sintering of the hard alloy parts is performed in a hydrogen atmosphere or preferably in vacuum. During temperature increase to the end temperature, a liquid phase occurs depending on the hard alloy type, or in other words depending on the composition and desired structure, and between 1,600 K. and 1,900 K. It finally leads to pore-free or almost pore-free tight sintering.
The production sequence of the hard alloy sintering which has been described above is only exemplary and typical for the production of plungers and matrixes for the ultrahigh pressure technique.
A critical part of the process for producing the above mentioned large hard metal parts is the presintering, especially the degrowing and the degassing. Moreover, it is also true for the first portion of the final sintering in which the open pores still exist so that many reactive gases can reach them or the pores must be released from gases by vacuum.
The driving out of the wax or paraffin fractions the reactive gas exchange and the degassing must be performed in the large sinter bodies with fine-grain, substantially precompressed pressing structure over relatively long, multiple branched and very narrow passages. These conditions in small sinter bodies are relatively favorable due to the short path from the interior to the outer surface and do not pause substantial problems. In large sinter bodies there are however long passages which are not favorable. The time for the intermediate temperatures of the presintering must be therefore extended over proportionally to the size of the parts. The danger that despite the increased time for the degrowing, composition residues remain and lead to false carbon content or during the evaporation of the growth residues for forming pores increases with the size of the sinter cross-section. Moreover, the conditions for the reduction of residual oxides are unfavorable since the hydrogen supply and the deviation of the gaseous oxidation products are significantly hindered. Finally, the degassing of the pores performed by the outwardly acting vacuum is hindered in the interior of great cross-sections.
Since the large hard alloy sinter parts are provided for extreme loads so that in their maximum stressed cross-sections the elasticity region is more or less exceeded, the smallest quality or strength reduction influences their technical loading ability and thereby is decisive for their operational reliability.